JOEL - There wasn't really any single event that prompted the Church leaders to be inspired to write the Proclamation on the Family, except of course maybe a prompting from God. They had observed for a long time the degradation of marriage and family that seemed to be happening at a greater rate throughout the world now than in times past. And they felt it was important to let the world know the LDS position on the subject and to give the world a God inspired warning about what would happen if the trend continued.
They also wanted to not only warn people about the problem but also to actually provide a solution that everyone could follow no matter what religion they belong to.
An excellent description and discussion about this proclamation was delivered by Henry B. Eyring. More information about the proclamation can also be found at this site.